Despite unnerving rivals with the MP4-28's pace in the first test at Jerez a fortnight ago, Button remains concerned that its performance is fluctuating too much.
"It's been very tricky to understand where the car is," he said.
"At times the car feels good, at other times it doesn't.
"A lot of that is basically understanding the car that we have and understanding the way it's working and making sure it's working the way we expect it. Some of the time that hasn't been the case.
"But there are times when the car's working pretty well. Today on hards I did a 1m22.8s, which I don't think is a bad lap with the fuel that we had on board. And Checo [Sergio Perez] did a 1m21.8s on softs yesterday, which is also pretty good.
"At Jerez we showed pace, but we also showed that we don't have pace at other times.
"We don't know where we are and there's no point trying to compare us to other people. We've got to sort out our own issues and get the best out of the car."
While some rivals, including champion Red Bull, took evolutionary approaches for 2013, McLaren went for a largely new design concept.
Button is still confident this will pay off in the long run, but acknowledged the team could start its campaign at a disadvantage.
"If we started this year with last year's car with a few changes to it, and we'd developed that car into 2013, I think we could've started with a very good car at the first race," he said.
"Whether it would be better than the MP4-28, I don't know. I think it would be a very strong car.
"But I think after three or four races you would realise that you're at the end of the development curve with it.
"For us, it's always important to change the car over the winter so we can see a new development direction over the course of the year.
"It's a long season at 19 races. It's about being strong over the whole season, not just the first couple of races.
"I am still hoping that we are competitive in Melbourne. It's a circuit I love and where I've been strong for several years.
"But there's a possibility that teams that kept the same car as last year and just developed it could also be strong."